// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();
// holds the remote server address 
var serverAddress = "validate.php";
// when set to true, display detailed error messages
var showErrors = true;
// initialize the validation requests cache 
var cache = new Array();

// creates an XMLHttpRequest instance
function createXmlHttpRequestObject() {
	// will store the reference to the XMLHttpRequest object
	var xmlHttp;
	// this should work for all browsers except IE6 and older
	try {
		// try to create XMLHttpRequest object
		xmlHttp = new XMLHttpRequest();
	} catch (e) {
		// assume IE6 or older
		var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
				"MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0",
				"MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLHTTP");
		// try every id until one works
		for ( var i = 0; i < XmlHttpVersions.length && !xmlHttp; i++) {
			try {
				// try to create XMLHttpRequest object
				xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
			} catch (e) {
			} // ignore potential error
		}
	}
	// return the created object or display an error message
	if (!xmlHttp)
		displayError("Error creating the XMLHttpRequest object.");
	else
		return xmlHttp;
}

// function that displays an error message
function displayError($message) {
	// ignore errors if showErrors is false
	if (showErrors) {
		// turn error displaying Off
		showErrors = false;
		// display error message

		alert("Error encountered: \n" + $message);
		// retry validation after 10 seconds
		setTimeout("validate();", 10000);
	}
}

// the function handles the validation for any form field
function validate(inputValue, fieldID) {
	// only continue if xmlHttp isn't void
	if (xmlHttp) {
		// if we received non-null parameters, we add them to cache in the
		// form of the query string to be sent to the server for validation
		if (fieldID) {
			// encode values for safely adding them to an HTTP request query string
			inputValue = encodeURIComponent(inputValue);
			fieldID = encodeURIComponent(fieldID);
			// add the values to the queue
			cache.push("inputValue=" + inputValue + "&fieldID=" + fieldID);
		}
		// try to connect to the server
		try {
			// continue only if the XMLHttpRequest object isn't busy
			// and the cache is not empty
			if ((xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
					&& cache.length > 0) {
				// get a new set of parameters from the cache
				var cacheEntry = cache.shift();
				// make a server request to validate the extracted data
				xmlHttp.open("POST", serverAddress, true);
				xmlHttp.setRequestHeader("Content-Type",
						"application/x-www-form-urlencoded");
				xmlHttp.onreadystatechange = handleRequestStateChange;
				xmlHttp.send(cacheEntry);
			}
		} catch (e) {
			// display an error when failing to connect to the server
			displayError(e.toString());
		}
	}
}

// function that handles the HTTP response
function handleRequestStateChange() {
	// when readyState is 4, we read the server response
	if (xmlHttp.readyState == 4) {
		// continue only if HTTP status is "OK"
		if (xmlHttp.status == 200) {
			try {
				// read the response from the server
				readResponse();
			} catch (e)

			{
				// display error message
				displayError(e.toString());
			}
		} else {
			// display error message
			displayError(xmlHttp.statusText);
		}
	}
}

// read server's response 
function readResponse() {
	// retrieve the server's response 
	var response = xmlHttp.responseText;
	// server error?
	if (response.indexOf("ERRNO") >= 0 || response.indexOf("error:") >= 0
			|| response.length == 0)
		throw (response.length == 0 ? "Server error." : response);
	// get response in XML format (assume the response is valid XML)
	responseXml = xmlHttp.responseXML;
	// get the document element
	xmlDoc = responseXml.documentElement;
	result = xmlDoc.getElementsByTagName("result")[0].firstChild.data;
	fieldID = xmlDoc.getElementsByTagName("fieldid")[0].firstChild.data;
	// find the HTML element that displays the error
	message = document.getElementById(fieldID + "Failed");
	// show the error or hide the error
	message.className = (result == "0") ? "error" : "hidden";
	// call validate() again, in case there are values left in the cache
	setTimeout("validate();", 500);
}

// sets focus on the first field of the form
function setFocus() {
	document.getElementById("txtUsername").focus();
}
